This invention relates to blends of phthalate esters, and halogenated benzene compounds, and more particularly, relates to compositions of branched chain phthalate esters and halogenated benzene compounds and to a method for controlling the dielectric constant of phthalate ester blends.
The compositions of this invention have utility in devices and methods which require stable fluids having a high dielectric constant. For example, the blends of alkyl branched chain phthalate esters and chlorinated benzene compounds may be used as a dielectric liquid to improve electrical capacitors. The uses of the fluid blends of the present invention are described in greater detail below.
U.S. Pat. No. 3,363,156, Cox, discloses and claims various types of dielectric liquid impregnated electrical capacitors. The dielectric liquid composition described in Cox is a chlorinated hydrocarbon, more particularly, a halogenated aromatic hydrocarbon and specifically a chlorinated diphenyl. The chlorinated diphenyl impregnants for electrical capacitors were commercially available under the trademark Aroclor, a trademark of the Monsanto Company, a specific example being Aroclor 1242 or Aroclor 1016. The chlorinated diphenyls, referred to as PCB's, have recently have associated with ecological problems, and accordingly, there is a continuing search for new and improved impregnants in the electrical field.
In U.S. Pat. No. 3,754,173, Eustance, the substance of which is incorporated herein by reference, there is disclosed an epoxide stabilized liquid aromatic ester impregnant which obviates many of the PCB ecological disadvantages. For example, Eustance discloses and claims the use of an epoxide with dioctyl phthalate. More importantly, however, is the fact that this unique combination has proved effective in electrical capacitors in many of the applications formerly served with chlorinated diphenyl capacitors.
In high voltage capacitor applications it was found that the aromatic esters in accordance with the Eustance patent will provide very high corona start voltages (CSV) and therefore favorably compare with the chlorinated diphenyl liquids for capacitors. However, a further important voltage level or criteria of a capacitor is referred to as the corona extinction voltage (CEV) of a capacitor. The corona start voltage and the corona discharge may commence in a capacitor and be extinguished respectively during a rising and decreasing voltage level across the capacitor. In high voltage power factor correction capacitors, the corona extinction voltage for the ester impregnants has been found to be significantly less than the corona extinction voltage for the chlorinated diphenyl type impregnants.
Additive liquids used to minimize the foregoing problems have been found in many cases to have adverse affects on other characteristics of the impregnant, such as, lowering the dielectric constant (DK), whereas the primary desire is to stabilize or increase the DK and the CEV. In addition to the foregoing problems, many of the ester impregnants, particularly those of higher molecular weights, are otherwise desirable for capacitor impregnants but have an increased viscosity which creates a problem in essentially completely impregnating a capacitor in accordance with the teachings of the above noted Cox patent.
There have been many attempts in the prior art to overcome these problems by resorting to mixtures of esters with chlorinated and other compounds. At first these efforts were directed to the use of chlorinated compounds for mixing with other fluids, notably esters. Those mixtures provided little, if any, unobvious improvements in electrical characteristics. The dielectric constants of the mixtures followed the well known mixing rules in that the dielectric constant of mixtures do not exceed the dielectric constant of the components, and other electrical characteristics were unimportant. Mixing became an acceptable practice for physical modifications of such characteristics as viscosity and solubility. See, for example, U.S. Pat. No. 3,112,356, Cohen, disclosing chlorinated aromatic compounds for mixing with monocarboxylic acid esters, without any particular reason for doing so, or U.S. Pat. No. 2,096,550, Jira, disclosing the use of an ester of phthalic acid in combination with halogenated cyclic hydrocarbon compounds and particularly the chlorinated diphenyl compounds.
More recently, the prior art has made an effort to find mixtures with improved electrical characteristics, such as dielectric constants and CSV. See, for example, U.S. Pat. No. 3,811,077, Munch, where a very high dielectric constant sulfone fluid is mixed with an ester to raise the dielectric constant of the mixture above that of the ester, but not of the sulfone, or British Pat. No. 1,428,741 where an ester is mixed with a diphenyl oxide to provide a fluid comparable to chlorinated diphenyl with a resultant dielectric constant of the mixture less than that of the ester.
While the chlorinated compounds generally were most desirable for mixing with esters for physical modifications, the polychlorinated diphenyl compounds were not desirable, and the reactions of non-diphenyl chlorinated compounds with the more desirable esters were relatively unknown, unpredictable, or contributed little besides physical changes. As a consequence, there was little, if any, motivation to proceed with such mixtures, and the art turned to various hydrocarbons for newer fluids. However, stabilization of, or an increase in, the corona extinction voltage and dielectric constant of a mixture which includes an acceptable chlorine compound with a desirable ester, would be most important.